Ductwork Under House

HouseMedic · Fri Apr 06, 2007 8:32 pm

I have a modular home 23 x 40 that was made in two haves and then set on a block foundation as a craw space.(approx. 3.5ft) The ductwork is completely shot and I am going to replace it. The original was aluminum 3 1/4" x 14" run on both haves with a crossover duct. Do you think I could run one line down the side with the heater and just use 6" round to go to the other side right to the registers? I also would like to know about using something like 8" ductwork for the main since finding the original size is hard to come by. Any thought or info would be greatly appreciated.

Thanks,
Ron

Yanita · Fri Apr 06, 2007 8:49 pm

Hi Housemedic,

Welcome to the site!

I am sure there are plenty here that will answer your question, but you should also post it in the HVAC forum. Robert is pretty much the man with the info on HVAC.

~Yanita~

Greg · Fri Apr 06, 2007 8:55 pm

Ron, I am going to try moving this to the heating section. Robert is better versed in this type of question. Greg

hvac1000 · Fri Apr 06, 2007 9:05 pm

((The original was aluminum 3 1/4" x 14" run on both haves ))

You need to maintain the same number of main square inches of duct. You now have a total of 183 square inches of duct. That is both sides times two.
Our homes are marginal at best on duct work supply for forced air furnaces. You will need to maintain the duct exactly the way it was originally installed to maintain the number of square inches and for even heat and A/C distribution. Smaller flex ducts will not work because you are tapping just the one duct at it does not contain the required number of square inch area to service the home.

It would be best to use the same layout as the manufacturer intended but I would seal the new duct with sealer to protect form loosing air at the seams. Usually the factory does not seal the duct.

Good luck with your project and that duct size you have should be available from the HVAC supply house.

Do right so you can enjoy your home in comfort.

hvac1000 · Fri Apr 06, 2007 9:06 pm

Woops. Greg would you move my answer also. Thank you.

Yanita · Fri Apr 06, 2007 9:08 pm

Hi Guys,

I am not Greg, but I will put this in the HVAC forum.

Apologies HVAC for not mentioning you as an HVAC person. LOL, since the new site I have not had time to follow the HVAC forums.

Here we go..

~Yanita~

HouseMedic · Sat Apr 07, 2007 7:21 pm

First, Sorry for putting in wrong area. Thanks for moving.
HVAC1000 Thanks for you response. I guess I will be putting it back the way it was. I just didn't know if it could be done another way. I can get the ductwork made but so far I have checked four supply houses and no one has that size. I am going to check more in the area where my home is located, there is allot more manufactured homes in that area. I guess it doesn't matter if it's galv. or aluminum. The original was aluminum. How about where the register branches come off the main duct they just used the joist cavity, Should I or could I use some size of duct work for them?
Thanks again for your help.
Ron

hvac1000 · Sat Apr 07, 2007 9:54 pm

Galv is actually better that aluminum. You can use takeoffs from the main trunk line to the hole. If it is a tall takeoff you may have to cut it down to fit.

Robert · Sun Apr 08, 2007 6:03 pm

I just wanted to add that if you go with galvanized, be sure you seal and insulate the belly tight and keep that way.

If you get any tears or openings, it increases the warm air contact to duct and creates sweating.

That will cause rust to form on the main duct if galv. .

Thanks,
Robert

HouseMedic · Tue Apr 10, 2007 5:29 pm

Robert, Would you try to find aluminum duct? Or just use the galvanized and completely seal the belly?

Ron

Robert · Tue Apr 10, 2007 7:40 pm

Whichever is available and affordable. Galv. must be sealed up tight in belly and remain that way to prevent rust formation.

Aluminum is easier as it is what is there and meets same design factors, so nothing changes.

Thanks,
Robert

HouseMedic · Mon Jul 30, 2007 9:55 pm

Hi All,
I am finally getting to the ductwork. I still have some questions tho.
Attached is a drawing of what I am pretty sure was the way that the original ductwork was. I am not sure where the crossover duct was connected. Should it be under the heater or moved up the duct? I guess I am going with the 3 1/4" x 14" main ducts but I think I am going to add take offs with 6" round going to the original floor registers. Other then the crossover duct position in the picture do you see anything that I should change? In the picture the ducts are light blue and the cross over duct is orange. My heater is in the middle with a circle drawn around it. Do you think this will be sufficient?

Thanks,
Ron

If you need to see the picture larger http://www.thehousemedic.com/images/Bea ... worklg.jpg

Robert · Mon Aug 06, 2007 3:04 am

Hi Ron,

Between this heat wave running me to death and buying a new home and having to work on current one and the new one, I'm behind on the questions some.

Anyway, the a/c went on new one and with it at 100* every day, gotta have a/c to work over there,LOL.

So, give me a couple days to look over the drawings and I'll see how it looks.

I can tell ya the crossover runs beneath or very close proximity to the furnace and then over to other half.

Thanks,
Robert

HouseMedic · Mon Aug 06, 2007 6:52 pm

Thanks Robert,

I know how it is to get very busy. We are having the heat here also. It's been killing me at work also. Of course when it's this hot all the work is always outside and never in the AC.

I was also wondering if it would be better to run just one trunk duct and just run longer 6" round duct over to the back rooms. The modular is on a block crawl space and it will not be separated anymore. Would this be just screwing things up?

Thanks,

Ron